Various forms of automatic steering systems have recently been proposed which automatically steer the vehicle according to the configuration of the road. The road information can be derived from a GPS system incorporated with a map of the area, and/or from a television camera which may detect the position of a white line marked on the road or the shape of the road. It is also possible to use other guidance systems for guiding the vehicle along the road. According to such road information, a target value of a dynamic variable such as yaw rate, lateral acceleration, and steering angle is computed, and the vehicle steering system is controlled in such a manner that the deviation between the target value of the dynamic variable and the actual value of the dynamic variable is minimized.
It is now customary for a motor vehicle to be equipped with a power steering system which assists the manual steering input applied to a steering wheel by using a hydraulic cylinder or an electric motor. FIG. 3 shows a typical electric power steering system. A steering wheel 21 is attached to an upper end of a steering shaft 22, and a lower end of the steering shaft 22 is connected to a pinion 24 via a connecting shaft 23 which is provided with universal joints 23a and 23b at two ends thereof. The pinion 24 meshes with a rack 27 which extends laterally of the vehicle body and is guided to move along a longitudinal axial line thereof. The two ends of a rack shaft 28 carrying the rack 27 are connected to knuckle arms 26 of right and left front wheels W via tie rods 25. To provide a power assist to this steering system, an electric motor 29 is coaxially combined with the rack 27 for axially actuating the rack 27 via a ball and nut mechanism 30.
A steering torque sensor 31 is provided in an appropriate part of the steering system to detect the magnitude of the steering effort applied to the steering wheel 21 by the vehicle operator. The electric motor 29 is controlled by a controller 32 which receives an output signal from the steering torque sensor 31 so that a desired target steering torque may be produced.
The automatic steering system and the power steering system described above have various common components, and a significant reduction in size, cost, and complexity of the overall system can be achieved by sharing these common components by the two systems. The simplification of the overall system also contributes to an improvement in the reliability of the operation of these systems.
However, when these two systems are simply combined so as to share various component parts, certain inconveniences may arise. For instance, when both the automatic steering system and the power steering system are in operation, a manual effort to change the driving lane of the vehicle gives rise to an actual dynamic variable such as yaw rate, lateral acceleration, and steering angle which deviates from the target value of the dynamic variable, and the system tends to resist the manual effort.
To resolve such an inconvenience, it has been previously proposed in Japanese patent laid open publication No. 7-205831 to allow an automatic steering mode and a power steering mode to be selected in a mutually exclusive manner, and to provide a certain time lag in a switch-over between the two modes. However, according to this previous proposal, the time lag produces a blank period during which there is no steering control so that the transition from one mode to the other is not very smooth, and the vehicle operator may experience an undesirable sudden change in the handling of the vehicle.